2,3,5,6 or symmetrical tetrachloropyridine is used for the preparation of herbicides and pesticides. Numerous processes have been employed in the past for the preparation of 2,3,5,6-tetrachloropyridine. Several processes that utilize non-electrolytic methods are disclosed in U.S. Pat. Nos. 3,186,994; 3,538,100; 3,993,654; 4,127,575; 4,256,894; 4,259,495; 4,281,135; 4,321,389; 4,322,538; and 4,327,216. Among those methods, the methods disclosed in U.S. Pat. Nos. 3,993,654; 4,259,495; 4,321,389; and 4,322,538 are methods wherein 2,3,5,6-tetrachloropyridine is produced from pentachloropyridine.
U.S. Pat. No. 3,993,654 discloses a process for preparing 2,3,5,6-tetrachloropyridine by heating pentachloropyridine in an aqueous medium under pressure, with agitation, in the presence of zinc and hydrogen chloride. U.S. Pat. No. 4,259,495 discloses a process for the preparation of 2,3,5,6-tetrachloropyridine by dechlorinating pentachloropyridine in an ester solvent, in the presence of an ammonium salt of an inorganic or an organic acid. U.S. Pat. No. 4,321,389 discloses a process for the production of 2,3,5,6-tetrachloropyridine by transhalogenating pentachloropyridine by a bromide salt in a polar, aprotic solvent at a temperature from about 100.degree. C. to about 140.degree. C., followed by selectively debrominating with hydrogen, in the presence of a noble metal catalyst and an acid acceptor. U.S. Pat. No. 4,322,538 discloses a process for producing 2,3,5,6-tetrachloropyridine by reacting pentachloropyridine with iodide ions and a proton donor and a polar, aprotic solvent at a temperature from about 100.degree. C. to 200.degree. C.
Some of the disadvantages of the aforementioned processes utilizing pentachloropyridine as the starting material to produce 2,3,5,6-tetrachloropyridine include the disposal problems associated therewith, i.e., zinc chloride produced in the method disclosed in the U.S. Pat. No. 3,993,654; the need for separating the products from the solvents in the processes where such solvents are used; and the costs associated with the use of catalysts and the use of solvents.
In addition to the aforementioned non-electrolytic processes used for the preparation of 2,3,5,6-tetrachloropyridine from pentachloropyridine, and electrolytic process is disclosed in U.S. Pat. No. 3,694,332, wherein a pentahalopyridine is electrolytically reduced to prepare a 2,3,5,6-tetrahalopyridine. The starting halogenated pyridine is dissolved in a suitable solvent containing an electrolyte, the solution is added to an electrolytic cell and current is passed through the cell until the desired degree of reduction is obtained. The cell utilized is a conventional one with or without a diaphragm. Preferred electrolytes are neutral or acidic salts. Several solvents could be employed depending on the reaction being considered.
Some of the disadvantages of the method disclosed in the U.S. Pat. No. 3,694,332 arise because of the use of the solvent. Those disadvantages include the reduction of the concentration of the electrolyte whereby the electrochemical operation of the reaction is hampered; the need for separating the product from the solvent following the electrolytic reaction; the complication of the operation; the increase in energy consumption; and increase in the potential of environmental pollution.
The present invention discloses an electrolytic reaction wherein pentahalopyridine is reduced to 2,3,5,6-tetrahalopyridine without utilizing an organic solvent, whereby all the disadvantages associated with the use of such solvent in U.S. Pat. No. 3,694,332 are eliminated.
These and other advantages of the present invention will become apparent from the following description.